In recent years, an optical recording medium has been widely used in large volume data files and the like as a medium capable of reading with laser light.
This optical recording medium includes an ROM type medium conducting only reproduction, such as a video disc, a compact disc and CD-ROM, a WO type medium capable of conducting additional recording, and a magneto-optical type or phase changeable type medium capable of conducting rewriting.
In the ROM type medium, information is formed in a substrate at a time of injection molding as a prepit, and a metal reflective layer is formed on the substrate as a thin layer in order to reflect light for reproduction.
In the WO type medium, information is recorded and reproduced utilizing the phenomenon that light transmission/reflective ratio characteristics vary with irradiation of laser light.
In the magneto-optical type medium in which rewriting can be freely carried out, information is recorded by heat magnetic reverse of a magnetic zone in a vertical magnetized film, and is reproduced by the Kerr effect. In the phase changeable type medium, recording and reproduction of information is effected utilizing a change in the light transmission/reflective ratio characteristics between a crystalline layer and an amorphous layer.
The shape or the light recording medium includes a compact disc medium of the one-side-recording type in which a optical recording layer, a protective layer and so on are provided on a substrate, and which is used as a single plate. A WO type medium, a magneto-optical type medium, a phase changeable type medium and so on of the double-side-recording type in which 3.5 inch magneto-optical discs and their media are bonded together by the use of an adhesive, are also included.
The above-described magneto-optical recording media are required to have a certain degree of reflectance because reproduction is conducted with laser light. In order to satisfy this requirement, a metal reflective layer in the form of a thin film is used in the ROM type medium. Even in the WO type medium, which uses a metal-based recording medium, a metal reflective layer is provided as a matter of course to obtain a suitable reflectance and to improve a signal quality.
Moreover, in the optical type recording medium, a metal reflective layer is used to improve a signal quality at a time of reading out.
It is important that a material for the above metal reflective layer to have a high light reflectance be useful in an optical recording medium. Au, Ag, Al, and the like are used for this.
Of these metals, Al is widely used from a viewpoint of cost. It is formed as a thin film on a substrate along with a recording layer, a protective layer and so on by a vacuum film forming method such as a sputtering method or a vacuum deposition method.
However, the durability of an Al reflective layer is low. That is, in the case of a one-side-recording type medium, oxygen or moisture directly from the air or permeating through a substrate and a protective layer causes oxidation and corrosion. This decreases the reflectance and breaks the thin film. Moreover, halogen and low molecular weight materials that remaining in a substrate and a protective layer made of an organic resin cause corrosion, thereby producing a problem particularly where long term durability is required.
Although the double-side-recording type recording medium provides better shielding from the air than the one-side-recording type recording medium, it inevitably suffers from the problems described above. Particularly in the case of the intimate contact structure in which two media are bonded together with an organic resin protective layer in the inside configuration, corrosion due to water permeating through an interface between the bonded media may occur. In addition, corrosion due to halogen and low molecular weight materials remaining in the adhesive produces a practically unneglegible problem.
In order to increase anti-corrosion properties of an Al reflective layer to be used in an optical recording medium, a method for making alloys of Al and various other elements have been proposed. For example, an alloy of Al and Si, Mg or Mn is used as a reflective film or layer of a magneto-optical recording medium to prevent a recording layer from deterioration due to oxidation (as described in JP-A-62-239349) (the term "JP-A" as used herein means an "unexamined published Japanese patent application); a method in which an alloy of Al and Si, Cr, Mg or Mn is used as a reflective film or layer of a magneto-optical recording medium, thereby preventing corrosion of the reflective film itself (as described in JP-A-62-295232); a method in which an alloy of Al and Ta is used as a reflective film of a magneto-optical recording medium, thereby preventing corrosion of the reflective film itself and a recording layer, and deterioration of characteristics (as described in Japanese Patent Application No. 63-78594 and JP-A-64-4938); and a method in which an alloy of Al and Ti is used for the same purpose as above (as described in JP-A-62-13774) have been proposed.
However, the methods in which the above aluminum alloys are used do not produce sufficiently satisfactory effects.
In addition, as a method of increasing anti-corrosion properties of an Al thin film against acid, amorphousizing an alloy of Al and Ta or Nb has been proposed (as described in JP-A-64-25934). However, this method does not produce satisfactory effects, because even if a film is produced by using bulky metal or by vacuum film forming, the thickness is greater than an Al thin film used in an optical recording medium by not less than one order of magnitude.
Consequently, an effective means to increase anti-corrosion properties of an Al reflective layer to be used in an optical recording medium has not been found.
In particular, no effective means has been proposed to prevent the corrosion characteristic of the intimate bonding structure of an Al reflective layer in a double-side-recording type optical recording medium using a hot melt adhesive or an organic resin protective layer so that corrosion due to halogen and low molecular weight materials remaining in an organic substance remains a problem.